The power amplifier is a key technology in portable radiotelephone design. In cellular telephones, the power amplifier has a large impact on the available talk time. This is because the power amplifier consumes a significant amount of power relative to the other circuitry within the cellular telephone. One parameter defining how much power the power amplifier consumes is the power amplifier efficiency.
Power amplifiers whose DC supply voltages are continuously varied to match signal level requirements for improving efficiency throughout a predetermined range of input signal levels are generally known. One such example is disclosed in U.S. Pat. No. 4,442,407 entitled, "TWO LOOP AUTOMATIC LEVEL CONTROL FOR POWER AMPLIFIER", issued to Thomas R. Apel on Jun. 11, 1982. In '407 the power amplifier is operated with improved efficiency by modulation of the RF amplifier DC supply voltage in response to a comparison between a signal, corresponding to the weighted sum of the magnitude of the power amplifier load current and supply voltage, and the amplitude of the modulation signal.
The system disclosed in '407, however, does nothing to address another important performance parameter of power amplifiers used for cellular telephone systems-transmitted adjacent and alternate channel power. In cellular telephone systems, the radiated adjacent channel power can cause interference in other cellular channels, thus causing a degradation in overall system performance. The adjacent and alternate channel power parameters are even more critical in cellular systems employing linear modulation schemes such as Interim Standard (IS)-136 Time Division Multiple Access (TDMA) and IS-95 Code Division Multiple Access (CDMA). By optimizing the power amplifier for efficiency with no regard to adjacent and alternate channel power performance, the power amplifier can fail the adjacent and alternate channel power specifications for a particular cellular system.
A method to simultaneously increase the linearity and efficiency of power amplifiers is disclosed in U.S. Pat. No. 5,101,172 entitled, "LINEAR AMPLIFIER", issued to Yukio Ikeda, et al., on Dec. 1, 1990. In '172 the drain voltage is controlled by a DC/DC converter to follow the amplitude level of the output signal. This increases power amplifier efficiency but introduces amplitude modulation (AM) and phase modulation (PM) distortion. Input and output envelope detectors are thus employed in conjunction with phase and amplitude comparators in order to introduce pre-distortion to counteract the distortion introduced by the power amplifier. This system requires accurate tracking of the power amplifier distortion, which can be difficult. In addition, the multiple couplers and phase/amplitude compare circuitry adds size and cost when used in a portable cellular telephone.
Another technique to minimize power amplifier distortion is disclosed in U.S. Pat. No. 4,348,644 entitled "POWER AMPLIFYING CIRCUIT WITH CHANGING MEANS FOR SUPPLY VOLTAGE," issued to Shingo Kamiya on Mar. 24, 1980. In '644 a power amplifying circuit detects the crest factor (e.g. peak-to-average ratio) of the output signal of a power amplifier. When the crest factor is large, the power amplifier supply voltage is raised. Conversely, when the crest factor is small, the supply voltage is lowered. Thus, when more power amplifier supply voltage is needed to handle the high peak-to-average ratio, the supply voltage is raised. Conversely, when there is a small peak-to-average ratio, the supply voltage is lowered. The high peaks are thus faithfully reproduced by raising the supply voltage, and the power loss is reduced by raising and lowering the power amplifier supply voltage as necessary.
The '644 technique is useful in electronic systems to amplify musical signals. In this type of application, faithful reproduction of the musical signal is necessary in order to provide for acceptable fidelity. However, the '644 technique does not address the need for trading off fidelity versus efficiency in a manner necessary to provide for cost effective and highly efficient portable radiotelephones.
Accordingly, there is a need to provide more accurate and comprehensive control of the adjacent and alternate channel power transmitted by a power amplifier. There is a further need for the power amplifier to operate efficiently for linear modulation schemes. A method of trading off linearity and efficiency is needed for power amplifiers used in portable radiotelephones. There is also a need to control power amplifier adjacent channel power, alternate channel power, and efficiency performance to compensate for part-to-part variations, temperature variations, load impedance variations, and frequency variations. There is also a need to control the power amplifier average transmit power while controlling the power amplifier linearity and efficiency.